If you need to add links to an article, or change that are already there, the Joomla editor provides a straightforward way to do it. In this example, I have links to "Hotel Information" and "Washington D.C.", and I want "Transportation" to be a link as well.

First I highlight the text that I want to act as a link. You can see below that I've highlighted "Transportation". Then I click one of the images in the toolbar above the editor that looks like a chain link. The one on the left is for links outside the side, and the one on the right is for links inside the site. I'm linking to something inside the site, so I'll use the one on the right (in the picture below, the mouse pointer is about to click it).

If you need to add a link to somewhere else on the web, you can use the other chain link button (without the colors), and just paste the address for the site into the box. Everything else works the same.

After I click the icon, the window below pops up. I need to fill in the "Link URL" box, and I like to fill in the "Title" box as well. The others are not important. 

I started typing the name of the section, and the editor automatically determined which part of the site I wanted to link to. So I click on "Transportation":

And it fills the other values in for me. Then I click OK and it closes the window and saves it for me.

If I need to edit a link, I click to put the cursor anywhere inside the word and click the same button (the chain link). The window comes back up and I can change the link.

Now if I visit the site, I can see the new link:

Screenshot of site with new link